Sunday, 27 December 2015

BASIC OF HELICAL GEAR

BASIC OF HELICAL GEAR

Before understanding the concept of helical gear, we
must understand the each term used in gear drive mechanism and we have already discussed
it i.e. various terminology used in gear in our previous post i.e. gear terminology. So before going ahead, i will request to my readers to find out
the gear terminology in our previous post as well as the concept of spur gear,
advantage, disadvantage and applications of spur gears in our last post i.e.
Basics of spur gear, advantage, disadvantage and applications.

We will discuss here the basics of helical gears and
finally we will see the various advantages, disadvantages and applications of
helical gears.

So
let us start our discussion about basics of helical gears

Helical gears are used to transmit the power and
motion between two non- intersecting shafts. Helical gears are similar with
spur gears but teeth will be cut over the surface of helical gear at an angle with
the axis of gear, while in case of spur gears teeth were made straight and
parallel with the axis of gear.

Helical gears will be prepared with teeth of both type
of hand i.e. Right hand teeth and left hand teeth. Following figure shows the spur
gear, helical gear of left hand teeth and right hand teeth.

Image courtesy: Google

If the thumb of our left hand is following the
inclination of tooth of gear, that helical will be termed as left hand helical
gear.

If the thumb of our right hand is following the
inclination of tooth of gear, that helical will be termed as right hand helical
gear.

Also read

Helical gears could be used for connecting parallel shafts
and non parallel shafts also. When we consider connecting and transferring
power and motion between two parallel shafts with the help of helical gears, in
that case opposite hand teeth will mesh with each other i.e. right hand helical
gear teeth will lean with left hand helical gear teeth.

While if we are considering for connecting and
transferring power and motion between two non-parallel shafts, acting at right
angle with each other, with the help of helical gears, in that case same hand
teeth will mesh with each other i.e. right hand helical gear teeth will lean
with right hand helical gear teeth and vice versa. Helical gears those are used
for connecting two non-parallel shafts will be termed as crossed helical gears.

We have assumed above that both shafts are acting at
right angle to each other and hence we have indicated that mating helical gear
will have same hand teeth but suppose if both shafts are not acting at right
angle then in that case both gears might have similar or opposite hand teeth
according to the shaft angle.

Helical gears those are used for connecting and
transferring motion and power between two parallel shafts will operate quite
smoothly and silently as compared with spur gears. There will be gradual
contact between helical gear teeth i.e. engagement will start at one end of a
gear tooth with other mating gear tooth and will spread gradually and
continuously as both mating gears rotate. Helical gears are preferred to use in
automobile applications due to its characteristic of silent operation and
capability to work smoothly under heavy load.

Pair of helical gears those are used for connecting
and transferring motion and power between two parallel shafts must have similar
helix angle, pitch and pressure angle but both helical gears must have opposite
hand teeth i.e. if driving helical gear is of right hand teeth then driven
helical gear must be of left hand teeth.

There are two important terms i.e. helix angle and
shaft angle those are quite important to understand here, let us see first
shaft angle, i will request to my readers to correct me if i am wrong anywhere
by giving your feedback or comments in comment box below.

Shaft
Angle

In case of crossed helical gears, shaft angle will
be defined as the angle between the ends of shafts those are rotating in
opposite directions.

Image courtesy: Google

Helix
angle

Helix angle of a helical gear will be defined as the
angle between axis of gear and an imaginary line, which will be tangent to the gear.
Helix angle will be in a range of 0 to 90 degree for a helical gear.

When a pair of helical gear meshes with each other,
there will be creation of axial thrust load on gear due to helix angle of gear
teeth and therefore gearbox designer has to select such bearings those are able
to absorb and support this axial thrust load.